This research seeks to determine the role that extracellular matrix (ECM) plays in the aging of skeletal muscle. Skeletal muscle tissue is maintained via the regenerative capacity of the tissue's resident stem/progenitor cells known as satellite cells or muscle progenitor cells (MPCs). With age, the regenerative capacity of MPCs is diminished along with the quantity and quality of skeletal muscle tissue. In determining how MPC regenerative capacity is lost, one must consider that both cell-intrinsic and cell- extrinsic changes can impact MPCs. Factors extrinsic to the cell include environmental components such as soluble factors, neighboring cells, and ECM, each of which can experience changes with age. The research described here investigates the impact of ECM age on the ability of MPCs to carry out the regenerative maintenance of skeletal muscle tissue. I hypothesize that age-related changes to skeletal muscle ECM result in misregulation of the regenerative mechanisms used by MPCs to maintain skeletal muscle. Two complementary and mutually reinforcing Specific Aims have been designed to test this hypothesis. In the first, young and old MPCs will be cultured on surfaces coated with young and old skeletal muscle ECM extract to determine how ECM age impacts MPC proliferation and differentiation in a static and relatively simple ex vivo environment. In the second Specific Aim, slices of young and old muscle will be decellularized and serve as ECM scaffolds onto which young and old MPCs will be seeded. The seeded scaffolds will be put into bioreactors and subjected to repeated stretch/relaxation cycles to stimulate muscle formation. In this way, the ability of young and old skeletal muscle ECM to support the ex vivo generation of muscle from young and old MPCs in a three-dimensional, dynamic environment can be compared. PUBLIC HEALTH RELEVANCE: The loss of skeletal muscle mass and strength that occurs during the aging process, known as sarcopenia, represents a major health issue facing the elderly population. Sarcopenia leads to functional decline, loss of independence, and physical disability. Furthermore, the loss of skeletal muscle tissue is associated with increased mortality even after adjusting for comorbid disease. Little is known about the role of ECM in sarcopenia. The research described here will be among the first to isolate ECM age as a variable in the study of skeletal muscle aging. Specifically, it will directly investigate how the aging of skeletal muscle ECM is related to the diminished regenerative capacity of MPCs observed in sarcopenia.